The invention disclosed and claimed herein is generally directed to an array of X-ray detector cells, such as are used in a computed tomography (CT) imaging system, wherein the cells of the array are maintained in a substantially isothermal, or constant temperature, condition. More particularly, the invention pertains to a detector array of the above type which is provided with means for rapidly transferring heat from higher temperature locations to lower temperature locations, in order to acheive the isothermal condition. Even more particularly, the invention pertains to a detector array of the above type wherein the heat transfer means are comparatively simple and inexpensive, and operate with a high degree of efficiency.
In a CT imaging system or scanner, a gantry ring rotates an X-ray tube around a patient or other object of scanning. X-radiation projected by the tube, which is not absorbed by intervening patient body structure, is sensed by respective detectors of a detector array which is also mounted to the gantry ring. In certain classes of CT products, such as those manufactured by the General Electric Company, the assignee herein, the array comprises solid state detectors or detector cells which generate electric signals corresponding to the sensed radiation. The signals are coupled to a data acquisition system, and data acquired thereby is in turn coupled to an image processor which reconstructs an image of patient body structure or other object of interest. In a common arrangement, the detector cells are mounted to the gantry ring or plate by means of two curved rails, which trap or hold respective detector cells between them to form an array having an arcuate configuration.
In the design of a solid state CT detector array, it is essential to maintain respective X-ray detectors of the array at a fixed temperature, in order to maintain a constant gain at the detectors. If the temperature of the detectors changes, their respective electric signal outputs, for a given dose of X-radiation, will also change. In addition, it is very desirable to hold all the detectors at substantially the same temperature (within a few degrees) in order to prevent movement of adjacent wires or other structure, which could partially block some of the detectors from receiving X-rays. Moreover, the detector mounting rails also support a pair of collimator plates for each detector cell. The collimator plates of a given detector cell are selectively spaced apart, to determine the incident X-ray radiation received thereby. If the detector mounting rails experience thermal deflection, i.e., motion or flexure caused by a temperature gradient along the rails, the spacing between some of the collimator plates may change. This, in turn, will effect the amounts of radiation received by the corresponding detectors.
In view of the problems caused by temperature variations, efforts have been made in the past to maintain an X-ray detector array in an isothermal condition, that is, to maintain a substantially constant temperature at all detectors of the array and along the rails thereof. To this end, heating elements have been placed at selected locations with respect to the rails, and heating strips are placed along the rails to distribute heat. However, it has been found that even with these arrangements, holding a uniform temperature on the rails, under all scanning conditions, tends to be very difficult. The rails rely on thermal conduction to move heat from one region to another, since the heating elements do not supply the appropriately distributed heat load for all possible detector operating conditions. Heat transfer in currently used rail designs requires that a temperature gradient be developed, and may proceed too slowly for present operational needs. Moreover, the temperature gradient in the detector mounting rails can change under different scanning configurations. In addition, the rails can be deflected by thermal gradients that are developed in the gantry plate to which the rails are attached. This plate currently is not thermally controlled. The gantry plate has power supplies mounted to it that can produce large thermal gradients, and these gradients may change as the gantry plate rotates during scanning.